Decoupled responses of above- and below-ground beta-diversity to nitrogen enrichment in a typical steppe.

Increased atmospheric nitrogen (N) deposition affects biodiversity in terrestrial ecosystems. However, we do not know whether the effects of N on above-ground plant ß-diversity are coupled with changes occurring in the soil seed bank. We conducted a long-term N-addition experiment in a typical steppe and found that above-ground ß-diversity increased and then decreased with increasing N addition, whereas below-ground ß-diversity decreased linearly. This suggests decoupled dynamics of plant communities and their soil seed bank under N enrichment. Species substitution determined above- and below-ground ß-diversity change via an increasing role of deterministic processes with N addition. These effects were mostly driven by differential responses of the above-ground vegetation and the soil seed bank ß-diversities to N-induced changes in environmental heterogeneity, increased soil inorganic N concentrations and soil acidification. Our findings highlight the importance of considering above- and below-ground processes simultaneously for effectively conserving grassland ecosystems under N enrichment.

Mechanisms of biodiversity loss under nitrogen enrichment: unveiling a shift from light competition to cation toxicity.

The primary mechanisms contributing to nitrogen (N) addition induced grassland biodiversity loss, namely light competition and soil cation toxicity, are often examined separately in various studies. However, their relative significance in governing biodiversity loss along N addition gradient remains unclear. We conducted a 4-yr field experiment with five N addition rates (0, 2, 10, 20, and 50 g N m-2 yr-1) and performed a meta-analysis using global data from 239 observations in N-fertilized grassland ecosystems. Results from our field experiment and meta-analysis indicate that both light competition and soil cation (e.g. Mn2+ and Al3+) toxicity contribute to plant diversity loss under N enrichment. The relative importance of these mechanisms varied with N enrichment intensity. Light competition played a more significant role in influencing species richness under low N addition (≤ 10 g m-2 yr-1), while cation toxicity became increasingly dominant in reducing biodiversity under high N addition (>10 g m-2 yr-1). Therefore, a transition from light competition to cation toxicity occurs with increasing N availability. These findings imply that the biodiversity loss along the N gradient is regulated by distinct mechanisms, necessitating the adoption of differential management strategies to mitigate diversity loss under varying intensities of N enrichment.

Opposing responses of temporal stability of aboveground and belowground net primary productivity to water and nitrogen enrichment in a temperate grassland.

Changes in water and nitrogen availability, as important elements of global environmental change, are known to affect the temporal stability of aboveground net primary productivity (ANPP). However, evidences for their effects on the temporal stability of belowground net primary productivity (BNPP), and whether such effects are consistent between belowground and aboveground, are rather scarce. Here, we investigated the responses of temporal stability of both ANPP and BNPP to water and nitrogen addition based on a 9-year manipulative experiment in a temperate grassland in northern China. The results showed that the temporal stability of ANPP increased with water addition but decreased with nitrogen addition. By contrast, the temporal stability of BNPP decreased with water addition but increased with nitrogen enrichment. The temporal stability of ANPP was mainly determined by the soil moisture and inorganic nitrogen, which modulated species asynchrony, as well as by the stability of dominant species. On the other hand, the temporal stability of BNPP was mainly driven by the soil moisture and inorganic nitrogen that modulated ANPP of grasses, and by the direct effect of soil water availability. Our study provides the first evidence on the opposite responses of aboveground and belowground grassland temporal stability to increased water and nitrogen availability, highlighting the importance of considering both aboveground and belowground components of ecosystems for a more comprehensive understanding of their dynamics.

Environment and microbiome drive different microbial traits and functions in the macroscale soil organic carbon cycle.

Soil microbial traits and functions play a central role in soil organic carbon (SOC) dynamics. However, at the macroscale (regional to global) it is still unresolved whether (i) specific environmental attributes (e.g., climate, geology, soil types) or (ii) microbial community composition drive key microbial traits and functions directly. To address this knowledge gap, we used 33 grassland topsoils (0-10 cm) from a geoclimatic gradient in Chile. First, we incubated the soils for 1 week in favorable standardized conditions and quantified a wide range of soil microbial traits and functions such as microbial biomass carbon (MBC), enzyme kinetics, microbial respiration, growth rates as well as carbon use efficiency (CUE). Second, we characterized climatic and physicochemical properties as well as bacterial and fungal community composition of the soils. We then applied regression analysis to investigate how strongly the measured microbial traits and functions were linked with the environmental setting versus microbial community composition. We show that environmental attributes (predominantly the amount of soil organic matter) determined patterns of MBC along the gradient, which in turn explained microbial respiration and growth rates. However, respiration and growth normalized for MBC (i.e., specific respiration and growth) were more linked to microbial community composition than environmental attributes. Notably, both specific respiration and growth followed distinct trends and were related to different parts of the microbial community, which in turn resulted in strong effects on microbial CUE. We conclude that even at the macroscale, CUE is the result of physiologically decoupled aspects of microbial metabolism, which in turn is partially determined by microbial community composition. The environmental setting and microbial community composition affect different microbial traits and functions, and therefore both factors need to be considered in the context of macroscale SOC dynamics.

Nutritional changes in trees during drought-induced mortality: A comprehensive meta-analysis and a field study.

Both macronutrients and micronutrients are essential for tree growth and development through participating in various ecophysiological processes. However, the impact of the nutritional status of trees on their ability to withstand drought-induced mortality remains inconclusive. We thus conducted a comprehensive meta-analysis, compiling data on 11 essential nutrients from 44 publications (493 independent observations). Additionally, a field study was conducted on Pinus sylvestris L. trees with varying drought-induced vitality loss in the "Visp" forest in southern Switzerland. No consistent decline in tree nutritional status was observed during tree mortality. The meta-analysis revealed significantly lower leaf potassium (K), iron (Fe), and copper (Cu) concentrations with tree mortality. However, the field study showed no causal relationships between nutritional levels and the vitality status of trees. This discrepancy is mainly attributed to the intrinsic differences in the two types of experimental designs and the ontogenetic stages of target trees. Nutrient reductions preceding tree mortality were predominantly observed in non-field conditions, where the study was conducted on seedlings and saplings with underdeveloped root systems. It limits the nutrient uptake capacity of these young trees during drought. Furthermore, tree nutritional responses are also influenced by many variables. Specifically, (a) leaf nutrients are more susceptible to drought stress than other organs; (b) reduced tree nutrient concentrations are more prevalent in evergreen species during drought-induced mortality; (c) of all biomes, Mediterranean forests are most vulnerable to drought-induced nutrient deficiencies; (d) soil types affect the direction and extent of tree nutritional responses. We identified factors that influence the relationship between tree nutritional status and drought survival, and proposed potential early-warning indicators of impending tree mortality, for example, decreased K concentrations with declining vitality. These findings contribute to our understanding of tree responses to drought and provide practical implications for forest management strategies in the context of global change.

Changes in productivity partitioning induced by precipitation extremes increase inaccuracy of grassland carbon estimation.

The fraction of net primary productivity (NPP) allocated to belowground organs (fBNPP) in grasslands is a critical parameter in global carbon cycle models; moreover, understanding the effect of precipitation changes on this parameter is vital to accurately estimating carbon sequestration in grassland ecosystems. However, how fBNPP responds to temporal precipitation changes along a gradient from extreme drought to extreme wetness, remains unclear, mainly due to the lack of long-term data of belowground net primary productivity (BNPP) and the fact that most precipitation experiments did not have a gradient from extreme drought to extreme wetness. Here, by conducting both a precipitation gradient experiment (100-500 mm) and a long-term observational study (34 years) in the Inner Mongolia grassland, we showed that fBNPP decreased linearly along the precipitation gradient from extreme drought to extreme wetness due to stronger responses in aboveground NPP to drought and wet conditions than those of BNPP. Our further meta-analysis in grasslands worldwide also indicated that fBNPP increased when precipitation decreased, and the vice versa. Such a consistent pattern of fBNPP response suggests that plants increase the belowground allocation with decreasing precipitation, while increase the aboveground allocation with increasing precipitation. Thus, the linearly decreasing response pattern in fBNPP should be incorporated into models that forecast carbon sequestration in grassland ecosystems; failure to do so will lead to underestimation of the carbon stock in drought years and overestimation of the carbon stock in wet years in grasslands.

Global pattern of organic carbon pools in forest soils.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Interannual fluctuations in precipitation shape the trajectory of ecosystem respiration along a grazing exclusion chronosequence in a typical steppe in Inner Mongolia.

Grazing exclusion (GE), as an effective strategy for revitalizing degraded grasslands, possesses the potential to increase ecosystem respiration (Re) and significantly influence the capacity of grassland soils to sequester carbon. However, our current grasp of Re dynamics in response to varying durations of GE, particularly in the context of precipitation fluctuations, remains incomplete. To fill this knowledge gap, we conducted a monitoring of Re over a 40-year GE chronosequence within Inner Mongolia temperate typical steppe across two distinct hydrologically years. Overall, Re exhibited a gradual saturation curve and an increasing trend with the duration of GE in the wet year of 2021 and the normal precipitation year of 2022, respectively. The variance primarily stemmed from relatively higher microbial biomass carbon observed in the short-term GE during 2022 in contrast to 2021. Moreover, the impacts of GE on the sensitivities of Re to moisture and temperature were intricately tied to precipitation patterns. increasing significantly with prolonged GE duration in 2022 but not in 2021. Our study highlights the intricate interplay between GE duration, precipitation variability, and Re dynamics. This deeper understanding enhances our ability to predict and manage carbon cycling within typical steppe in Inner Mongolia, offering invaluable insights for effective restoration strategies and climate change mitigation.

Mowing aggravates the adverse effects of nitrogen addition on soil acid neutralizing capacity in a meadow steppe.

Soil acidification induced by reactive nitrogen (N) inputs is a major environmental issue in grasslands, as it lowers the acid neutralizing capacity (ANC). The specific impacts of different N compound forms on ANC remain unclear. Grassland management practices like mowing and grazing can remove a considerable amount of soil N and other nutrients, potentially mitigating soil acidification by removing N from the ecosystem or aggravating it by removing base cations. However, empirical evidence regarding the joint effects of adding different forms of N compounds and mowing on ANC changes in different-sized soil aggregates is still lacking. This study aimed to address this knowledge gap by examining the effects of three N compounds (urea, ammonium nitrate, and ammonium sulfate) combined with mowing (mown vs. unmown) on soil ANC in different soil aggregate sizes (>2000 µm, 250-2000 µm, and <250 µm) through a 6-year field experiment in Inner Mongolia grasslands. We found that the average decline in soil ANC caused by ammonium sulfate (AS) addition (-78.9%) was much greater than that by urea (-25.0%) and ammonium nitrate (AN) (-52.1%) as compared to control. This decline was attributed to increased proton (H+) release from nitrification and the leaching of exchangeable Ca2+ and Mg2+. Mowing aggravated the adverse effects of urea and AN on ANC, primarily due to the reduction in soil organic matter (SOM) contents and the removal of exchangeable Ca2+, K+, and Na + via plant biomass harvest. This pattern was consistent across all aggregate fractions. The lack of variation in soil ANC among different soil aggregate fractions is likely due to the contrasting trend in the distribution of exchangeable Ca2+ and Mg2+. Specifically, the concentration of exchangeable Ca2+ increased with increasing aggregate size, while the opposite was true for that of exchangeable Mg2+. These findings underscore the importance of considering the forms of N compounds when assessing the declines of ANC induced by N inputs, which also calls for an urgent need to reduce N emissions to ensure the sustainable development of the meadow ecosystems.

The two sides of resistance-resilience relationship in both aboveground and belowground communities in the Eurasian steppe.

The ongoing nitrogen (N) deposition has led to profound changes in aboveground and belowground ecosystems. However, the stability of plant and soil microbial community toward N addition in terms of resistance and resilience is less understood. We established a long-running field trial (2008-2018) in a series of N applications in combination with a mowing and fencing (unmown) treatment in a semiarid steppe. We assessed the resistance via ongoing N treatment of one subplot and the resilience via discontinuing N treatment in another to promote natural recovery since 2014. Plant resistance was negatively correlated with N application rate, while microbial resistance was independent of N rate. Mowing significantly reduced plant resistance and resilience, reduced soil microbial resistance but improved its resilience. Generally, plants are more resilient but less resistant to N than soil microbes. The two sides of resistance-resilience relationship were revealed: trade-offs exist between resistance and resilience for both plants and microbes at the community level; and trade-offs between resistance and resilience cannot be scaled down to species/group level. This study provided an important theoretical basis for the recovery and conservation of semiarid steppe and new insight into resistance-resilience relationship.

Identifying thresholds of nitrogen enrichment for substantial shifts in arbuscular mycorrhizal fungal community metrics in a temperate grassland of northern China.

Nitrogen (N) enrichment poses threats to biodiversity and ecosystem stability, while arbuscular mycorrhizal (AM) fungi play important roles in ecosystem stability and functioning. However, the ecological impacts, especially thresholds of N enrichment potentially causing AM fungal community shifts have not been adequately characterized. Based on a long-term field experiment with nine N addition levels ranging from 0 to 50 g N m-2 yr-1 in a temperate grassland, we characterized the community response patterns of AM fungi to N enrichment. Arbuscular mycorrhizal fungal biomass continuously decreased with increasing N addition levels. However, AM fungal diversity did not significantly change below 20 g N m-2 yr-1 , but dramatically decreased at higher N levels, which drove the AM fungal community to a potentially unstable state. Structural equation modeling showed that the decline in AM fungal biomass could be well explained by soil acidification, whereas key driving factors for AM fungal diversity shifted from soil nitrogen : phosphorus (N : P) ratio to soil pH with increasing N levels. Different aspects of AM fungal communities (biomass, diversity and community composition) respond differently to increasing N addition levels. Thresholds for substantial community shifts in response to N enrichment in this grassland ecosystem are identified.

5300-Year-old soil carbon is less primed than young soil organic matter.

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the "priming effect" (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO2 efflux cannot be disregarded due to its huge pool size.

Different nitrogen saturation thresholds for above-, below-, and total net primary productivity in a temperate steppe.

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below-ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above-ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide-ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m-2 year-1 ) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (TA ) and NPP (TN ) was at the rate of 13.11 and 6.70 g N m-2 year-1 , respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m-2 year-1 , resulting in much lower TN than TA . Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super-saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above-ground might have over-estimated the positive effects of N deposition on primary productivity.

Enhanced foliar 15 N enrichment with increasing nitrogen addition rates: Role of plant species and nitrogen compounds.

Determining the abundance of N isotope (δ15 N) in natural environments is a simple but powerful method for providing integrated information on the N cycling dynamics and status in an ecosystem under exogenous N inputs. However, whether the input of different N compounds could differently impact plant growth and their 15 N signatures remains unclear. Here, the response of 15 N signatures and growth of three dominant plants (Leymus chinensis, Carex duriuscula, and Thermopsis lanceolata) to the addition of three N compounds (NH4 HCO3 , urea, and NH4 NO3 ) at multiple N addition rates were assessed in a meadow steppe in Inner Mongolia. The three plants showed different initial foliar δ15 N values because of differences in their N acquisition strategies. Particularly, T. lanceolata (N2 -fixing species) showed significantly lower 15 N signatures than L. chinensis (associated with arbuscular mycorrhizal fungi [AMF]) and C. duriuscula (associated with AMF). Moreover, the foliar δ15 N of all three species increased with increasing N addition rates, with a sharp increase above an N addition rate of ~10 g N m-2  year-1 . Foliar δ15 N values were significantly higher when NH4 HCO3 and urea were added than when NH4 NO3 was added, suggesting that adding weakly acidifying N compounds could result in a more open N cycle. Overall, our results imply that assessing the N transformation processes in the context of increasing global N deposition necessitates the consideration of N deposition rates, forms of the deposited N compounds, and N utilization strategies of the co-existing plant species in the ecosystem.

Functional structure mediates the responses of productivity to addition of three nitrogen compounds in a meadow steppe.

Atmospheric nitrogen (N) deposition is altering grassland productivity and community structure worldwide. Deposited N comes in different forms, which can have different consequences for productivity due to differences in their fertilization and acidification effects. We hypothesize that these effects may be mediated by changes in plant functional traits. We investigated the responses of aboveground primary productivity and community functional composition to addition of three nitrogen compounds (NH4NO3, [NH4]2SO4, and CO[NH2]2) at the rates of 0, 5, 10, 20 g N m-2 yr-1. We used structural equation modeling (SEM) to evaluate how functional structure influences the responses of productivity to the three N compounds. Nitrogen addition increased community-level leaf chlorophyll content but decreased leaf dry matter content and phosphorus concentration. These changes were mainly due to intra-specific variation. Functional dispersion of traits was reduced by N addition through changes in species composition. SEM revealed that fertilization effects were more important than soil acidification for the responses of productivity to CO(NH2)2 addition, which enhanced productivity by decreasing functional trait dispersion. In contrast, the effects of (NH4)2SO4 and NH4NO3 were primarily due to soil acidification, influencing productivity via community-weighted means of functional traits. Our results suggest that N forms with different fertilizing and acidifying effects influence productivity via different functional traits pathways. Our study also emphasizes the need for in situ experiments with the relevant N compounds to accurately understand and predict the ecological effects of atmospheric N deposition on ecosystems.

Macrofaunal control of microbial community structure in continental margin sediments.

Through a process called "bioturbation," burrowing macrofauna have altered the seafloor habitat and modified global carbon cycling since the Cambrian. However, the impact of macrofauna on the community structure of microorganisms is poorly understood. Here, we show that microbial communities across bioturbated, but geochemically and sedimentologically divergent, continental margin sites are highly similar but differ clearly from those in nonbioturbated surface and underlying subsurface sediments. Solid- and solute-phase geochemical analyses combined with modeled bioturbation activities reveal that dissolved O2 introduction by burrow ventilation is the major driver of archaeal community structure. By contrast, solid-phase reworking, which regulates the distribution of fresh, algal organic matter, is the main control of bacterial community structure. In nonbioturbated surface sediments and in subsurface sediments, bacterial and archaeal communities are more divergent between locations and appear mainly driven by site-specific differences in organic carbon sources.

Nitrogen addition and mowing had only weak interactive effects on macronutrients in plant-soil systems of a typical steppe in Inner Mongolia.

Effective management of macronutrients is pivotal in the optimization and provisioning of ecosystem services in grassland areas, particularly in degraded grasslands. In such instances where mowing and nitrogen (N) fertilization have emerged as predominant management strategies, nutrient management is especially important. However, the precise effects of these concurrent practices on the distribution of macronutrients in plant-soil systems remain unclear. Here we evaluated the effects of 12 years of N addition (2, 10, and 50 g N m-2 year-1) and mowing on the concentrations and pools of six macronutrients (i.e., N; phosphorus P; sulfur S, calcium Ca, magnesium Mg, and potassium K) in three plant components (aboveground plants, litter, and belowground roots) at the community level and in the soil in a typical steppe in Inner Mongolia. Our results revealed that N addition generally raised the N concentration in the entire plant-soil system, regardless of whether plots were mowed. Higher N addition (10 and 50 g N m-2 year-1) also led to higher concentrations of P (+22%, averaging two N addition rates), S (+16%), K (+22%), Ca (+22%), and Mg (+24%) in plants but lower concentrations of these nutrients in the litter. Similar decreases in K (-9%), Ca (-46%), and Mg (-8%) were observed in the roots. In light of the observed increases in vegetation biomass and the lack of pronounced changes in soil bulk density, we found that the ecosystem N enrichment resulted in increased pools of all measured macronutrients in plants, litter, and roots (with the exception of Ca in the roots) while concurrently decreased the pools of P (-20%, averaging two higher N addition rates), S (-12%), K (-10%), Ca (-37%), and Mg (-19%) in the soil, with no obvious effect of the mowing practice. Overall, mowing exhibited a very limited capacity to alleviate the effects of long-term N addition on macronutrients in the plant-soil system. These findings highlight the importance of considering the distribution of macronutrients across distinct plant organs and the dynamic nutrient interplay between plants and soil, particularly in the context of long-term fertilization and mowing practices, when formulating effective grassland management strategies.

Applied phosphorus is maintained in labile and moderately occluded fractions in a typical meadow steppe with the addition of multiple nutrients.

Phosphorus (P) is a limiting nutrient second only to nitrogen (N) in the drylands of the world. Most previous studies have focused on N transformation processes in grassland ecosystems, particularly under artificial fertilization with N and atmospheric N deposition. However, P cycling processes under natural conditions and when P is applied as an inorganic P fertilizer have been understudied. Therefore, it is essential to examine the fate of applied P in grassland ecosystems that have experienced long-term grazing and, under certain circumstances, continuous hay harvest. We conducted a 3-year field experiment with the addition of multiple nutrient elements in a typical meadow steppe to investigate the fate of the applied P in various fractions of P pools in the top soil. We found that the addition of multiple nutrients significantly increased P concentrations in the labile inorganic P (Lab-Pi) and moderately occluded inorganic P (Mod-Pi) fractions but not in the recalcitrant inorganic P (Rec-Pi) fraction. An increase in the concentration of total inorganic P was found only when P and N were applied together. However, the addition of other nutrients did not change P concentrations in any fraction of the mineral soil. The addition of P and N significantly increased the total amount of P taken up by the aboveground plants but had no effect on the levels of organic and microbial P in the soil. Together, our results indicate that the P applied in this grassland ecosystem is taken up by plants, leaving most of the unutilized P as Lab-Pi and Mod-Pi rather than being immobilized in Rec-Pi or by microbial biomass. This implies that the grassland ecosystem that we studied has a relatively low P adsorption capacity, and the application of inorganic P to replenish soil P deficiency in degraded grasslands due to long-term grazing of livestock or continuous harvest of forage in the region could be a practical management strategy to maintain soil P fertility.

Community response of arbuscular mycorrhizal fungi to extreme drought in a cold-temperate grassland.

Climate extremes pose enormous threats to natural ecosystems. Arbuscular mycorrhizal (AM) fungi are key plant symbionts that can affect plant community dynamics and ecosystem stability. However, knowledge about how AM fungal communities respond to climate extremes in natural ecosystems remains elusive. Based on a grassland extreme drought experiment in Inner Mongolia, we investigated the response of AM fungal communities to extreme drought in association with plant communities. The experiment simulated two types of extreme drought (chronic/intense) of once-in-20-year occurrence. AM fungal richness and community composition exhibited high sensitivity to extreme drought and were more sensitive to intense drought than chronic drought. This community sensitivity (i.e. decline in richness and shifts in community composition) of AM fungi can be jointly explained by soil moisture, plant richness, and aboveground productivity. Notably, the robustness of the plant-AM fungal community co-response increased with drought intensity. Our results indicate that AM fungal communities are sensitive to climate extremes, and we propose that the plant community mediates AM fungal community responses. Given the ubiquitous nature of AM associations, their climate sensitivity may have profound consequences on plant communities and ecosystem stability under climate change.

Plant genome size modulates grassland community responses to multi-nutrient additions.

Grassland ecosystems cover c. 40% of global land area and contain c. 40% of soil organic carbon. Understanding the effects of adding nutrients to grasslands is essential because they provide much of our food, support diverse ecosystem services and harbor rich biodiversity. Using the meadow steppe (grassland) study site of Inner Mongolia, we manipulated seven key nutrients and a cocktail of micronutrients to examine their effects on grassland biomass productivity and diversity. The results, explained in structural equation models, link two previously disparate hypotheses in grassland ecology: (1) the light asymmetry competition hypothesis and (2) the genome size-nutrient interaction hypothesis. We show that aboveground net primary productivity increases predominantly from species with large genome sizes with the addition of nitrogen, and nitrogen plus phosphorus. This drives an asymmetric competition for light, causing a decline in species richness mainly in species with small genome sizes. This dynamic is likely to be caused by the nutrient demands of the nucleus and/or the scaling effects of nuclear size on cell size which impact water use efficiency. The model will help inform the best management approaches to reverse the rapid and unprecedented degradation of grasslands globally.